IT BEARS all the hallmarks of a Cold War thriller - an athletic young recluse living in a chic London flat, a mysterious job working ''with codes'' and a violent death by unknown hands.

The murder of Gareth Williams - a 30-year-old communications officer seconded to the British Intelligence Service from GCHQ (Government Communications Headquarters), Britain's eavesdropping centre - has thrust a very private life into the glare of the public spotlight.

Williams was found in his top-floor Pimlico apartment on Tuesday night in a gruesome scene akin to a crime novel. His body, thought to have been decomposing for up to two weeks, was found stuffed into a large sports holdall, which had been placed in the bath.

Reports that he had been stabbed and dismembered have been denied but police appear to have confirmed that his mobile phone was found carefully laid alongside several SIM cards.

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Speculation that he had been murdered because of his work erupted immediately: for the past few years, Williams had worked with GCHQ, Britain's biggest listening post. The 5000-strong organisation in Cheltenham eavesdrops on global communications and monitors terrorist threats.

Yet during the past year, he had moved to London and it was colleagues at MI6, across the Thames from his elegant London flat, that reported him missing.

Nobody knows what he was working on but the city apartment has been described as a far cry from the granny flat he had lived in at Cheltenham and to which he was to return on September 3.

The Cheltenham flat was ''floral carpets, silk flowers and potpourri'', according to an account in London's Independent. His imminent return to the little flat near GCHQ has now been linked to the possibility that the reason for Williams's death may have been rather more banal, and the work of a jealous lover.

Yet the spy connection has continued apace amid revelations that the chic property in Pimlico - rented to a succession of people with Cheltenham GCHQ links - was owned by a company registered in the British Virgin Islands called New Rodina, the term for motherland in Russia.

Neighbours are well-heeled, including former Conservative home secretaries Sir Leon Brittan and Michael Howard.

Williams's long-time Cheltenham landlady, Jenny Elliott, 71, has spent the past 48 hours talking to London's feverish press, describing the perfect tenant over 10 years, quiet, reclusive, seemingly without friends and visited by family only once.

Girlfriends were never seen, although he worked late into the night: ''All I heard was a tape recorder being rewound or listened to over and over, although he must have had his earphones in because I couldn't make out what it was,'' she told The Independent.

''He was an extremely intelligent person but would not talk about his job as it was a secret. All he told me was it was something to do with codes,'' she told the Telegraph. ''It's a real tragedy what's happened. Gareth was a really nice guy who was polite and mild-mannered and wouldn't hurt a fly. He was a cycling fanatic and was forever off on some bike ride or another.''

Williams was a maths graduate who began a masters in advanced mathematics at St Catherine's College, Cambridge. His family described him as ''brilliant'' and he had attended a special primary school, and university one day a week while at secondary school. In 2001, during the notoriously gruelling masters degree, he failed an exam and left the course, immediately beginning working at GCHQ.

His parents, from Anglesey, flew back to Britain from a foreign holiday to identify their son's body. A first autopsy on Wednesday failed to establish a cause of death, AFP reported. Further tests are under way.

Police will describe the death only as ''suspicious and unexplained'' and inquiries are also focusing on his lifestyle. It is believed he was on annual leave while he was missing, perhaps explaining the time it took to report his disappearance.